1. Field of the Invention
The present invention relates to a scanning system converting apparatus and a method for the same, and more particularly to the technique in which a scanning system is converted from an interlace scanning system into a progressive scanning system when an image data which is coded by a coding system such as the MPEG2 (Moving Picture Expert Group 2) is decoded.
2. Description of the Related Art
As an image data which can be coded in accordance with MPEG2, there are an image data which is coded by an interlace scanning system which is used in monitors such as an NTSC TV monitor and an image data which is coded by a progressive scanning system which is used in monitors such as a monitor of a personal computer. When an image data coded in accordance with the MPEG2 is received through a communication line, a decoded image data must be outputted to the NTSC TV monitor in the interlace scanning system.
However, it is often requested that the image data is outputted in the interlace scanning system and, at the same time, is outputted to a monitor of the progressive scanning system such as the monitor of the personal computer. In such a case, it is necessary for a scanning system converting apparatus to convert the scanning system of the image data outputted from the interlace scanning system into the progressive scanning system.
However, conventionally, when the image data of a progressive scanning system originally is coded in accordance with the MPEG2, there is a problem in that the image quality is degraded, when the scanning system of the decoded image data is converted into the progressive scanning system by the scanning system converting apparatus, compared with the image quality before coding.
In the image signal of the interlace scanning system, the sampling position of the image data is shifted between a previous field and the current field in a vertical direction, as shown in relation of frame period and vertical axis of FIG. 1. Thus, it is necessary to determine whether or not there is a change in the images between the fields, when the scanning system of the image data is converted from the interlace scanning system into the progressive scanning system. For this determination, there is a method of calculating a difference between a signal interpolated from the upper line and the lower line and a signal in the previous field, as shown in FIG. 1. However, in this method, there is a case where the signal which is obtained by the interpolation greatly differs from an actual signal in the region with a high vertical resolution.
Also, as shown in FIG. 2, there is a method of determining a signal difference between the frames. However, when the image changes at a frequency which is higher than 30 Hz, the change can not be detected. Therefore, the image data whose scanning system is to be converted into the progressive scanning system is not identical with the image data before coding with any of the above methods.
Also, in the case contrary to the above, that is, when the image data of the interlace scanning system is coded in accordance with the MPEG2, the decoded image data is outputted in accordance with the progressive scanning system, and further the scanning system of the decoded image data is converted from the progressive scanning system into the interlace scanning system, there is a problem in that the image quality is degraded compared with the image quality before coding, as described above.
In addition, a video signal processing method is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-182111). In this reference, an interlace component signal of 270 MHz in a (4:2:2) signal format is inputted to the video processing circuit from an input terminal 1, and converted into a 10-bit signal of 27 MHz in parallel in the processing circuit. Also, first and second blocks of a progressive component signal are inputted from the input terminals 1 and 2. An interpolating operation is performed to the color components of the first and second signal blocks so as to generate a serial signal of 360 MHz in a (4:2:2:4) signal format. Then, the serial signal is converted into a 10-bit parallel signal of 36 MHz. Finally, these component signals are re-converted into the signal formats in the input.
Also, a video display system is described in Japanese Laid Open Patent Application (JP-A-Heisei 8-251504). In this reference, a video signal is received in a switch 106. A user can select, by the switch 106, at least one of auxiliary channel by which a main channel image and a special function image can be viewed, when he does not want to view the main channel image for the special function image. A main video channel data is converted in scanning system from an interlace scanning system into a progressive scanning system by a scanning system converter 216. A logic unit 212 processes an auxiliary channel data to format into a selected special function. The data is inputted into the scanning converter 216 and the special function image is displayed at a proper position to the main channel image.
Also, a TV signal transmission apparatus is described in Japanese Laid Open Patent Application (JP-A-Heisei 4-79685). In this reference, the TV signal transmission apparatus is composed of a first motion detecting section for detecting a motion in a frame of an image data of a progressive scanning system and a second motion detecting section for detecting a motion between frames of an image data of an interlace scanning system. When any motion is detected by the first motion detecting section and any motion is not detected by the second motion detection section, a fixed value is added to a transmission signal so that the motion can be detected based on the fixed value.